In a typical modern vehicular braking system a booster assembly provides braking assistance to an operator of a vehicle during a braking operation. The booster assembly is mechanically energized by the engine or by an electrically driven pump.
A vacuum booster assembly has historically been used to provide the booster functionality. Hydraulic boosters are also used. In either case (vacuum or hydraulic), the booster assembly provides a force to a primary piston of a master cylinder that is proportional to the amount of the brake pedal force that is applied by the operator. The master cylinder functionality may be provided by a separate master cylinder assembly, or the functionality may be integrated within the booster assembly.
In an integrated hydraulic booster-master cylinder combination, fluid pressure from a high pressure source is regulated by a booster valve in response to the brake pedal force. Fluid within a booster chamber, which is at the regulated pressure, is applied to the primary piston of the master cylinder portion of the combination in order to move the primary piston from a rest position (i.e., an unapplied position) to an applied position. The master cylinder and the primary piston define a primary chamber which is in fluid communication with a braking circuit. Moving the primary piston pressurizes fluid in the primary chamber, thereby pressurizing fluid in the braking circuit. In order to return the primary piston to the rest position or to an intermediate position between the applied position and the rest position, in response to a full or partial release of the brake pedal by the operator, the booster fluid that is within the booster chamber must be vented to reduce or eliminate the pressure applied to the primary piston by the fluid in the booster chamber. Without proper venting of the fluid in the booster chamber, the trapped fluid impedes or prevents the desired return of the primary piston to the unapplied position.
There is a need to provide an improved hydraulic boosted braking system that efficiently vents trapped fluid in the booster chamber to a reservoir to allow return of the primary piston with reduced impedance in the master cylinder in response to the brake pedal being released.